(1) Field of the Invention
The present invention relates to a method for producing multilayered glass-ceramic structures with copper-based conductors therein for use as ceramic circuit substrates provided with large scale integrated circuit (LSI) devices and the like.
(2) Description of the Prior Art
Multilayered ceramic circuit substrates comprise metal layers which act as patterned electrical conductors sandwiched between ceramic layers which act as electrical insulators. They are produced by laminating ceramic thin sheets including organic binder with printed electrical conductors thereon. The laminated structure is fired so that the binder material burns out and the ceramic particles coalesce, while the metal particles become a sintered dense metallic state.
Generally, a high-melting point metal such as molybdenum and tungsten or a noble metal such as gold is used to form the conductor layers. High-melting point metals enable high melting point alumina to be used as the insulator in such multilayered structures. Alumina is excellent in electrical insulation, thermal conductivity, and mechanical strength, and maintains these characteristics over long usage. However, its relatively high dielectric constant results in delayed signal transmission and a low signal-to-noise (S/N) ratio, and its relatively higher coefficient of thermal expansion, compared with silicon, results in mechanical defects when silicon chips are soldered thereon.
Of the high-melting point metals and noble metals, the noble metals are preferable in terms of their low electrical resistance, however, their use considerably increases overall production costs.
Therefore, it is desired to use a metal having a low electrical resistance and low cost, such as copper, as a conductor material and a ceramic material having a low coalescing temperature, such as glass-ceramic. To this end, it is necessary that the binder material contained in a glass-ceramic green sheet be depolymerized and burnt-out in an atmosphere and at a temperature wherein copper is not oxidized. When commonly known binder resins, such as polyvinyl butyral and polyvinyl acetate, are used in a glass-ceramic green sheet and are fired in a nitrogen atmosphere at a temperature up to 1150.degree. C., these resins are neither satisfactorily depolymerized nor thoroughly oxidized. Consequently, the fired multilayered structure becomes porous and has carbonous residue therein, thereby deteriorating the mechanical strength and electrical insulation of the structure.
Lester W. Herron et al. disclose in U.S. Pat. No. 4,234,367 a method for forming sintered glass-ceramic substrates containing multilevel, interconnected thick film circuit patterns of copper-based conductors obtained by firing in a controlled ambient of hydrogen and water at temperatures below the melting point of copper. Herron et al. teach heating the green laminate in an ambient of hydrogen and water in a ratio of H.sub.2 /H.sub.2 O of 10.sup.-4 to 10.sup.-6.5 to a burn-out temperature, about 700.degree. C. to 800.degree. C.
According to our calculation, when water vapor itself is used as such an ambient, water is thermally decomposed, so that the molar ratio of H.sub.2 /H.sub.2 O under a water pressure of 1 atmosphere is equal to about 10.sup.-6.8 at 700.degree. C. and about 10.sup.-6.0 at 800.degree. C.